This invention relates to a process for increasing the addition polymer content of a resinous reaction product in a liquid vehicle. More particularly, the invention is concerned with a process for in situ polymerization of ethylenic monomers in the presence of a water-reducible, epoxy-based aqueous dispersion. The invention is also concerned with products produced by these processes, and particularly, with water-reducible, epoxy-based coating compositions. The subject matter of the present patent application is related to the subject matter of other patent applications now being U.S. Pat. No. 4,212,781 and U.S. Pat. No. 4,285,847.
Graft polymers formed between epoxy resins and polymerized addition monomers including an acrylic acid are suggested in these patents and in certain prior art for use in coating compositions.
In the art prior to U.S. Pat. No. 4,212,781 and U.S. Pat. No. 4,285,847, so-called graft polymers were formed by an esterification reaction between an acidic addition monomer or polymer and an epoxy resin to form an ester graft. The techniques for making epoxy-based resins water-dispersible, through inversion with amines, and for cross-linking them with added aminoplast, are well known. However, ester adduct products have not been satisfactory and do not have good resistance to water since the adducts are susceptible to hydrolysis.
U.S. Pat. No. 4,212,781 discloses a process for resinous compositions for use in coating compositions particularly useful as sanitary coatings, that is, as coatings for cans to contain edible materials. Such can coating compositions are easy to apply in existing equipment, have excellent shelf stability, and after application and curing have excellent functional properties as coatings and do not impart any flavor to the food or beverage in the can. The process comprises preparing a curable resinous composition having an Acid Number of at least 30, by reacting together at 90.degree. C. to 130.degree. C. an aromatic diepoxide resin having a molecular weight above 1,000 and addition polymerizable monomer including 10% to 80% by weight acrylic acid, the diepoxide resin being present in sufficient quantity to provide from 30% to 90% by weight of the initial reaction mixture, in the presence of a free radical initiator of the benzoyl peroxide type. During this reaction there is simultaneous addition polymerization of the monomer through its ethylenic unsaturation and grafting of addition polymer to the diepoxide resin. The acid functionality of the reaction mixture is sufficiently high to effect stable dispersion of the product in a basic aqueous medium. As is more particularly pointed out in U.S. Pat. No. 4,212,781, the resinous reaction product produced contains three polymeric components, namely, the graft polymer, ungrafted diepoxide resin, and ungrafted addition polymer. The initial epoxy resin employed in the graft polymer production process can be terminated to eliminate part or all of the terminal epoxy groups to eliminate the possibility of ester grafting, as more particularly set forth in copending application Ser. No. 793,507, filed May 4, 1977.
In U.S. Pat. No. 4,285,847, a preferred embodiment sets forth a process for polymerizing in situ an added quantity of addition polymerizable monomer containing ethylenic (vinyl) unsaturation, in an aqueous dispersion of a resinous reaction product produced in accordance with a process of one of the earlier filed patent applications described above, that contains as the resinous component thereof a mixture of graft polymer, unreacted epoxy resin, and ungrafted addition polymer. The net result of that process is to reduce substantially the percentage of epoxy resin and increase substantially the percentage of polymerized addition polymerizable monomer. Another useful result is to increase the solids content of the composition.
It now has been found that an organic reducing agent, preferably a benzoin derivative advantageously avoids redox catalyst problems that can occur in the second stage monomer polymerization step wherein ethylenically unsaturated monomers are polymerized in the presence of the preformed resinous mixture which desirably contains (a) epoxy polymer, (b) addition polymer, and (c) grafted polymer of addition polymer grafted to the epoxy backbone. Too much catalyst can cause a protective coating to exhibit a blush whereas too little catalyst causes insufficient conversion and polymerization of the monomers. In accordance with this invention, an organic reducing agent, preferably benzoin, used in conjunction with peroxide catalyst advantageously avoids the blushing problem and further provides excellent conversion of the monomers during polymerization. This advantage is particularly important where large amounts of acrylic monomers are used whereby a film blush can now be avoided. A further advantage is achieved with coating compositions synthesized with benzoin wherein the cured coatings exhibit excellent resistance to strenuous detergent testing such as a 1% boiling Joy detergent test. These and other advantages will become more apparent by referring to the detailed description of the invention and the illustrative examples.